1. Field of the Invention
The present invention relates to a method for measuring the viscosity and/or elasticity of a liquid by which the viscoelasticity of the liquid to be tested can be accurately measured, and the viscosity or elasticity can be accurately determined from the viscoelasticity.
2. Description of the Related Art
Japanese Patent Publication No. 4083621 discloses a method for measuring a viscosity value (viscosity). By this method, a vibrator (such as a piezoelectric device) is resonated in a liquid to be tested, and the viscosity value (the viscosity) of the liquid is measured from the difference between the resonance frequency and either a low frequency value (a half-value frequency f1) lower than the resonance frequency value or a high frequency value (a half-value frequency f2) higher than the resonance frequency value.
Japanese Patent Publication No. 3348162 discloses a method for measuring the viscoelasticity of a liquid. By this method, the frequency and the amplitude that are varied by the inherent viscoelasticity of the liquid are detected from the vibration of a liquid tester in the liquid to be tested. The impedance of the liquid is determined from the frequency and the amplitude, and the viscosity value and the elasticity value are determined from the real part and the imaginary part of the impedance.
By the viscosity measurement method disclosed in Japanese Patent Publication No. 4083621, however, a viscosity value is measured by determining the real part of the impedance of the subject liquid based on the difference between the resonance frequency and either the half-value frequency f1 or the half-value frequency f2. By a measurement method involving only the real part of the impedance like this method, only the real part of complex viscosity can be determined. As a result, the viscosity value of a high viscosity liquid to be tested cannot be measured with accuracy, and it is impossible to measure both the viscosity value and the elasticity value that are required for determining the dynamic characteristics of the liquid.
By a measurement method involving determination of the impedance of a liquid from the frequency and the amplitude attributable to the inherent viscoelasticity of the liquid like the method disclosed in Japanese Patent Publication No. 3348162, or by a measurement method involving the real part of the impedance as the reciprocal of a sensor output voltage proportional to the amplitude, the measurement accuracy becomes poorer, since the amplitude becomes smaller as the viscosity of the tested liquid becomes higher.